Embodiments consistent with examples of this invention relate to nano-carbon hybrid structures, and more specifically, to mixtures of nanodiamonds and carbon nanotubes and/or onion-like carbon structures which can be combined with water or other solvents to form a suspension.
A number of structures involving interfaces between regions involving sp2 and sp3 bonded carbon can be formed. Each surface carbon atom on a bulk terminated diamond (111) surface is covalently bonded via three sp3 orbitals to three nearest neighbor atoms located in the first subsurface layer. The remaining sp3 orbital is oriented normal to the surface. Each surface atom also has six second nearest neighbors on the surface plane that form a hexagon with radius 0.252 nm (the second nearest neighbor distance in the diamond lattice). The radius of the hexagon formed by these surface carbon atoms is within ˜6% of the radius of a (6,0) carbon nanotube (“CNT”). This relatively close mismatch, together with the orientation of the surface sp3 orbitals, has been predicted to yield relatively strong covalent bonding between a bulk diamond (111) surface and a (6,0) CNT oriented normal to the surface. Sinnot (1999) and co-workers noted this potential for strong hybrid sp2-sp3 bonding between a bulk diamond surface and CNTs, and suggested that this hybrid configuration could in principle be used to produce a CNT-diamond composite with a Young's modulus in the direction of the tubule axis that exceeds the Young's modulus of diamond in the (111) direction by 4%, with the composite having a density that is only about 80% of the density of diamond.
Expanding on the bulk diamond (111)-(6,0) CNT bonding structure, Shenderova et al (2003) systematically characterized similar hybrid bonding configurations in which the strain energy due to geometric mismatch is sufficiently small and the orbital overlap sufficiently strong that a stable sp2-sp3 hybrid interface can be formed. These structures included both metallic and semiconducting nanotubes bonded to diamond clusters or substrates, theoretically leading to different types of heterojunctions for carbon-based nanoelectronic applications, including diodes, novel quantum dots, and robust field emitters. The simulations suggested that there is sufficient flexibility in the CNTs to accommodate most of the strain from geometrical mismatches at the bulk diamond-nanotube interfaces.
Experimental observations of hybrid interfaces in carbon structures have also been reported. In work by Kuznetsov et al., for example, nanometric closed curved graphitic structures with conical or tubular forms attached to a surface of a diamond particle were observed by high-resolution transmission electron microscopy (“HRTEM”) of diamond particles after high-temperature annealing. Avigal et al. and Ayres et al. have also reported simultaneous growth of hybrid structures of diamond crystallites and CNTs on the same substrate by plasma-enhanced chemical vapor deposition (CVD). In more recent work, Terranova et al. reported nanostructured carbon particles that are created in a chemical vapor deposition reactor via reactions between carbon powder and atomic hydrogen. A tubular inner structure consisting of bundles of single walled carbon nanotubes (SWCNTs) up to 15 um long, with an outer deposit consisting of well shaped diamond crystallites with diameters in the 20-100 nm range was observed.
Recently, Gruen et al. at Argonne National Laboratory achieved growth of ultrananocrystalline diamond (“UNCD”) films by CVD with carbon nanotubes incorporated into it during CVD process using catalyst precursors on the surface for nucleation and growth of CNT within the hybrid structure. It is predicted by Gruen that UNCD and UNCD/CNT composites might fulfill many of the requirements that could lead to high and highly unusual thermoelectric figures of merit.
References on Gruen's work include D. M. Gruen, and J. W. Elam, “Nanotube-Diamond Composites”, MRS Fall Meeting, Paper #Q2.3, (Dec. 1-5, 2003), D. Gruen, L. Curtiss, and P. Zapol, “Synthesis of Ultrananocrystalline Diamond/Nanotube Self-Composites by Direct Insertion of Carbon Dimer Molecules into Carbon Bonds”, European Diamond Conference, Toulouse, France, (Sep. 11-16, 2005), and Gruen in Chapter 5 in “Ultrananocrystalline diamond”, 2006, William Andrew Publisher